1. Field of the Invention
The present invention relates to an apparatus for the stabilizing of the direction of travel of watercraft, specifically a sword or fin for sailboards having a guide plate extending from the bottom side of the watercraft into the water, which guide plate has a symmetrical airfoil profile cross-sectional shape.
2. Description of the Prior Art
Such apparatuses which are given generally the shape of swords or tail fins give rise to various problems in connection with wind surfing boards and specifically at high velocities. The profile of the fin or sword, respectively, of sailboards or generally watercraft is designed similar as is the case with aircraft wings with the aim of reaching an as small as possible flow resistance and also of achieving an as large as possible uplift of such profile. The uplift of the sword or fin, respectively, counteracts the drifting of the wind surfer due to the force components of the sail. An optimal design of the lift as well as the optimal design of an as small as possible resistance depend, however, at a prevailing profile on the relative velocity of the flow of water relative to the profile. Accordingly, every profile has an optimal velocity, at which the largest possible lift may be achieved simultaneously with an as small as possible flow resistance in case of a laminar flow. Such optimization calculations can be solved in case of designing profiles of turbine blades, aircraft wings on the base of predetermined nominal velocities. Such solutions are, however, not suitable for the broad range of various velocities of such watercraft. A basic rule for bodies, around which a flow prevails is that the larger the relative velocity is the smaller the depth of profile must be, neglecting a change of the picture or aspect, respectively, ratio.
The difficulties present at watercraft of above mentioned design are increased due to the fact that the profiles defining the guiding wings are usually streamed against obliquely except in the case of a downwind run because due to the always present drifting watercraft and wind surfing boards move somewhat obliquely through the water. This leads now to the fact that relative to the two sides of the profile of these guide surfaces and in spite of the fact that they are given a symmetrical profile shape a positive pressure side and a negative pressure side exist. These pressure differences may thereby rise to such an extent that during high velocities such as can be presently reached specifically with wind surfing boards cavitations may be generated at the fins and at the sword which lead to a sudden veering of the course of the sailboards (see Windsurfing Magazine "Surf", July 1981, pages 74 ff.).